Automatic audio amplifier control



w. VAN B. ROBERTS AUTOMATIC AUDIO AMPLIFIER CONTROL Filed April 29, 1930March 12, 1935.

ATrdRNEY INVENTOR \vmmvm 5110551215 v Patented Mar.- 12, 1935 AUTOMATICAUDIO AMPLIFIER CONTROL Walter van B. Roberts, Princeton, N. 1.,assignor to Radio'Corporation of America, a corporation of DelawareApplication April 29, 1930, Serial No. 448,205,

11 Claims. (Cl. 179-171) My present invention relates to acousticreproducing devices, and more particularly to a combined automatic gainand tone control system adapted for use with audio frequency currents.In my co-pending application, Serial Number 448,206, filed April 29,1930, there has been disclosed a method of, and means for, controllingboth the total gain and the frequency response characteristics of anaudio amplifier by utilizing a single, multi-stage audio amplifier, andemploying a pilot, or regulating circuit for controlling the gainandtone of the amplifier. Briefly, this was accomplished by employingone of the stages of the amplifier for varying 15 the total gain of thelatter in a predetermined direction and in accordance with the averageaudio input intensity, and, additionally, utilizing another stage of theamplifier for varying the ratio of low to high audio frequencyamplificasible to simplify the aforementioned control system, thesimplified arrangement utilizing broadly.

a single stage of the amplifier for both gain and tone control.

Accordingly, it is one of the'main objects of my present invention toprovide a method of, and means for automatically controlling the toneand volume of a single multi-stage audio amplifier which consists inemploying one of the stages for varying the total gain of the amplifier,and, additionally, utilizing the same stage of the amplifier for varyingthe ratio of low to high audio frequency amplification.

Another important object of the invention is to provide a multi-stageaudio amplifier circuit, one stage of which circuit is employed forautomatically controlling both the tone and gain of the amplifier, thelatter being regulated in accordance with the acoustical conditionswithin the range of the amplifier reproducer.

Another object of the invention is to provide a single, multi-stageamplifier circuit, one stage of which circuit is employed forautomatically controlling the tone and gain of the amplifier, the saidstage including an interstage coupling device whose electricalcharacteristics are varied by a regulating circuit including a rectifierand a control means, both the tone and volume being 50 regulated inaccordance with the average audio input intensity.

Still other objects of the invention are to improve generally theefiiciency of audio amplifiers, and to provide a combined volume andtone control system for audio amplifiers which is not only reliable inoperation, but automatic in regulation, and economical in circuitarrangement.

The novel features which I believe to be characteristic of my inventionare set forth in par-' ticularity in the appended claims, the invention5 itself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicated one circuitorganization whereby my in- 10 vention may be carried into effect.

Referring to the accompanying drawing which presents diagrammatically acircuit embodying my invention, an audio amplifier circuit is shown, theinput leads 1 of which have impressed across 15 them modulated audioinput energy. The input energy may be derived from any desired sourcesuch as, the output circuit of the detector stage of a radio telephonereceiver, the output terminals of an electro-magnetic pick-up device orother electric phonograph device; the output terminals of the microphonearrangement of a public address system; the output terminals of thephoto-electric device of a sound motion picture arrangement; or, anyother source of modulated audio frequency currents. I have not shown anyof these sources in the drawing connected to leads 1, for the reasonthat they are well known to those skilled in the art; and, additionally,tov preserve simplicity of description throughout this specification.

, The audio frequency input, applied to the leads 1, is impressed acrossthe input terminals of an electron discharge device 1", the latterpreferably being of the screen grid type, the negative 35 leg of thefilament of the device including a resistance 1', and, additionally,being grounded as shown. The anodeof the tube 1 is supplied withpotential from the positive terminal of a source B, through the primarycoil 2 of an audio transformer T, the negative terminal .of the source Bbeing connected to the grounded terminal of resistance 1', The screenelement of the device 1" is positively biased from a source not shown.

The secondary coil 3 of the audio transformer T has its low potentialterminal connected to the negative terminal of a biasing source C, thepositive terminal of the latter being grounded. The high potentialterminal of the coil 3 is connected to the control electrode of a triode4. The anode of the latter triode has potential applied to it from asource B through an inductance coil 5, the negative terminal of thesource B being connected to the negative filament lead of the triode 4through a potentiometer resistance 6, both the negative filament leadand the resistance 6 being connected to the grounded positive ter-,.

minal of the source C A capacity 7 is connected between the negativefilament lead and the low potential terminal of the inductance coil 5,while another capacity 8 is connected in shunt with the capacity 7,between the negative filament lead and the high potential terminal ofthe inductance coil 5. An adjustable contactor 9 is arranged for slidingconnection to the resistance 8, the latter being connected to thecontrol electrode of a control electron discharge tube 10 by means of alead 11; a current source 12 being inserted in the lead 11, with thenegative terminal of the source being connecte to the adjustablecontactor 9.

The negative filament lead of the control tube 10 is connected to thegrounded terminal of the resistance 1, while the anode of said controltube receives potential from the source B through a lead 13 connected tothe anode terminal of the primary coil 2. An output electron dischargetube 14, which tube while shown herein as a triode may of course, be ascreen grid tube or a pentode output tube, has its control electrodeconnected by means of a lead 15 to the control electrode terminal of thesecondary coil 3. The negative filament lead of the tube 14 is grounded,and includes in series therein a resistance 16 shunted by a capacity 17.The anode of the output tube 14 receives potential from a source B1, thenegative terminal of the latter being connected to the grounded terminalof the negative filament lead. The anode of the output tube 14 and thepositive terminal of the source B1 are furthermore, connected to anywell known type of utilization means, such as phones, loud speaker andthe like.

In order to explain the operation of the control system disclosedherein, it should be pointed out that the audio amplifier consists of atleast two stages, one of which stages includes the device 1, while theother stage includes the output tube 14 .and the interstage couplingtransformer T. The tube 4 and its associated output circuit, the latterincluding the filter circuit comprising capacity 7, inductance 5 andcapacity 8, as well as the potentiometer 6, 9 serves as a rectifier. Thetube 10 functions as a control tube, and operates upon the interstagecoupling means T, which I have included as-a part of the second audiostage, to vary the gain and tone of the audio amplifier.

From another point of view, it will also be seen that the gain and tonecontrol of the audio amplifier is accomplished by automaticallyregulating the electrical characteristics of the coupling between atleast two audio stages, the regulating device including a rectifierassociated with the coupling output and a control tube associated withthe coupling input.

The rectified output of the tube 4 is smoothed out by the filter circuitreferred to, the latter being designed in such a manner that the currentoutput of the filter arrangement varies only slowly with variations inaudio input intensity; in other words, the rectifier circuit gives along period average of audio current energy.

' The potentials of batteries 0 and B are adjusted so that the potentialdrop across resistor 6 increases with increasing average voltage inputto tube 14. The voltage of source 12 and the position of slider 9 arethen arranged so that when the input to tube 14 is the maximum that canbe utilized by that tube without distortion, the platefilament impedanceof tube 10 is so high that the action of transformer T is not affectedby having the plate-filament resistance of tube 10 in parallel with thetransformer primary. When,'however, the input to tube 14 is small, thedrop across resistor 6 is small, and the grid bias of tube 10 is smallso that the shunting effect of the plate circuit of tube 10 acrosstransformer T is considerable. 'The position of slider 9 of coursedetermines the amount of control action, the control action becomingzero when the slider is at the grounded end of resistor 6. As shown mycoupling system between the two audio stages includes an iron coredtransformer T of current commercial type. It is known that a couplingsystem including such a transformer amplifies the high audio frequenciesmore than it does the lower frequencies due to its impedance falling offat the lower frequencies. When such a transformer is shunted by a fairlylow impedance, its impedance is greatly reduced and the amplification ofthe coupling system correspondingly reduced with the result that theamplification curve plotted against frequency input is lowered andbecomes practically parallel to the frequency axis. In other words, byshunting the transformer with a low impedance, I have not only reducedthe amplification of the coupling system at all frequencies but have atthe same time reduced the high frequency amplification substantiallymore than the low frequency amplification.

With high signal input voltage, the bias on the grid of tube 10increases, the plate-filament impedance of this tube is high and theresultant impedance of the coupling system is high. This conditionsecures a high gain and a greater am-.- plification of the high audiofrequencies than the low ones. With weak signal input voltage, thepotential of the grid of tube 10 rises, its platefilament impedancedecreases so that the effective impedance of the coupling system isconsiderably decreased. The gain of the amplifier is thus decreased andthe high and low frequencies amplified substantially equally. In otherwords the amplification of the low notes with respect to the high noteshas been increased automatically. In contra-distinction to an automaticvolume control system which maintains substantially constant outputcurrent, my invention permitsme to exaggerate the variation of thesignal strength (as represented in the output current) as the inputsignal varies.

From the aforegoing specific explanation of the operation of thecircuit, it will be seen that at high output levels, the controlelectrode of tube 10 becomes negative, and the tube 10, therefore, isinoperative to prevent the amplifier from havinga large gain, especiallyat high audio frequencies. At low output levels, the control electrodeof tube 10 becomes less negative, or even positive, and hence the lowanode resistance of the tube 10, which is in parallel with the coil 2cuts down the gain, particularly the high audio frequency gain, thusapproaching a fiat characteristic.

While I have indicated and described one arrangement for carrying myinvention into effect, it'will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationshown and described, but that many modifications may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What I claim is:

1. In combination with an audio amplifier comprising at least one audiofrequency coupling device, means controlling the coupling deviceimpedance connected with the said coupling, a rectifier for controllingsaid means in accordance with audio input level, for simultaneouslyvarying the tone. and volume of the ance, and a rectifier connected toanother portion of said coupling, and connected to vary the eifectiveimpedance of said tube for varying simultaneously the gain of saidamplifier and the ratio of low to high audio frequency amplification.

3. In combination a stage of audio amplification including an electrondischarge device, a second stage, including an electron dischargedevice,means for coupling the output of the first device to the input'of the second device, a rectifier having its input connected with theinput of the second device, a control tube/connected to the output ofsaid re'ctifierfid the coupling means for varying the resultantimpedance of the coupling means accordance with audio input level insuch a direction as to increase said impedance upon increase of theinput signal level.

4. In combination, an audio amplifier including a vacuum tube having aninput and an output circuit, a coil having acore of magnetic material insaid output circuit, a variable impedance connected in shunt to saidcoil and control means for automatically varying the value of saidimpedance in response to the amplifier input energy level whereby theamplification-- frequency characteristic of said amplifier may becontrolled.

5. In combination, anaudio amplifier including a vacuum tube having aninput and an output circuit, a source of audio frequency currentconnected to said input circuit, acoil having a core of magneticmaterial in said output circuit, arectifier having its input connectedacross said coil, a variable impedance connected to said coil and meansconnecting said rectifier and variable impedance whereby upon increaseof signal current in said input circuit the value ofsaid variableimpedance is automatically increased.

6. In combination with, an electrical network of the type including anamplifier of'alternating currents of a desired frequency, a load circuitand a coupling element connecting the amplifier and load circuit, arectifier adapted to produce directcurrent voltages from said currents,an electronic relay electrically connected across said coupling element,and an electrical connection between said rectifierand relay forimpressing said voltages on the latter the effective impedance of thecoupling element with currents of increasing amplitude.

"I. In combination with an electrical network of the type including anamplifier of alternating currents of a desired frequency, a load circuitand a coupling element connecting the amplifier and load circuit, arectifier connected across said coupling element adapted to producedirect current voltages from said currents, an electronic discharge tuberelay operatively associated with said coupling element, and anelectrical connection between said rectifier and relay for impressingsaid voltages onthe latter to vary its impedance in a direction toincrease the eflective impedance of the coupling element with currentsof increasing amplitude.

8. In combination with a. signal source and an electronic repeater, acouplin device between the source and repeater, the device having apredetermined frequency. response characteristic, means for developing adirect current voltage from signals varying with the signal amplitude,and means, responsive to said voltage, connected to said coupling devicefor accentuating the high frequency end of its characteristic andincreasing the signal transmission efiiciency of the device when thesignal amplitude increases.

9. In combination with a signal source and an electronic repeater, a'transformer coupling device between the source and repeater, the devicehaving a predetermined frequency response characteristic, means fordeveloping a direct current voltage from signals varying with the signalamplitude, and means, responsive to said voltage, connected to saidcoupling device for accentuating the high frequency end of itscharacteristic and increasing the signal transmission efficiency of thedevice when the signal amplitude increases. I r

10. In combination with a signal source and an electronic repeater, acoupling device between the source and repeater, the device having apredetermined frequency response characteristic, rectifier means fordeveloping a direct current voltage from signals varying with the signalamplitude, and means, responsive to said voltage, connected to saidcoupling device for accentuating the high frequency end of itscharacteristic and increasing the signal transmission efiiciency of thedevice when'the signal amplitude in- -;creases.' v

"11. Incombination with a signal source and an electronic repeater, acoupling device between the source .and repeater, the device having apredetermined frequency response characteristic, means for developing adirect current voltage from signals varying with the signal amplitude,and means, responsive to said voltage, connected in shunt to saidcoupling device for accentuating the high frequency end of" itscharacteristic-and increasing the signal transmission efiiciency of thedevice when the signal amplitude increases. to vary its impedance in adirection to increase wan'rna mi B. ROBERTS.

